The present invention relates generally to a speed monitor and more specifically to a digital tachometer.
On certain farm implements it is advantageous to be able to monitor several operating parameters. On a combine, for example, proper ground speed, header shaft speed, cleaning fan speed and engine speed are necessary for efficient removal of the crop from the field. Such problems as malfunctions, misadjustments, clogging and excessive loading can be quickly detected by monitoring the various speeds.
Although numerous digital tachometer devices are available, heretofore none have been completely satisfactory. Some require a separate instrument for each function and, as a result, are high in cost and require much panel space at the operator's station.
Accuracy is a problem with many tachometers. Some respond well during periods when the speed monitored is steady but have a slow response during acceleration and deceleration. Averaging techniques are often used which do not provide a true indication of speed and which can even, in certain situations, give an indication that speed is increasing when in fact the speed has just begun to decrease. If the device is made to respond quickly for accurate readings during periods of acceleration or deceleration, the display often is difficult to read during operation at steady speeds since small changes in the reading will cause constant change in the least significant digit. To prevent constantly changing digits at steady speeds, accuracy is often compromised.
Commonly, magnetic transducers detecting passage of teeth on a rotating member provide an alternating current input to the device which varies in amplitude with the angular velocity of the rotating member. At low speeds, problems of noise and sensitivity affect accuracy. Jitter often occurs because circuitry is used which counts the number of cycles or pulses occuring during a given clock period begun at random. This random counting can result in different counts for consecutive clock periods even if the speed remains constant. This causes the least significant digit to change constantly, which is annoying to the operator.
Ground speed measurements are usually derived from the rotational speed of a drive shaft and are affected by the size of the tires provided on the implement. If the tire size is changed, the ground speed indicated will be inaccurate. To correct for such changes, or alternatively to set the device to read in different units such as kilometers per hour rather than miles per hour, often requires an adjustment of a trimming potentiometer while a signal generator connected to the tachometer input simulates a signal for a given speed.